Potassium has been known for quite a long time to enhance the formation of longer chain and olefinic hydrocarbons over bulk Group VIII-metal catalysts, especially iron, which are used in the Fischer-Tropsch synthesis reaction. Potassium, however, facilitates the sintering of the Group VIII-metal surfaces. To minimize sintering, oxide supports such as SiO.sub.2 and Al.sub.2 O.sub.3 have been used to stabilize the Group VIII-metal surface areas. The use of high surface area supports tends, however, to physically separate the potassium and Group VIII-metals which diminishes the promotional effect of potassium (or rubidium). Thus, in the preparation of Group VIII-metal Fischer-Tropsch catalysts, a careful balance between promotional and stabilization additives must be achieved to maximize activity and selectivity patterns.
Bulk Group VIII-metal catalysts are very poorly dispersed and thus possess very low surface/volume ratios. Supported Group VIII-metal catalysts, however, contain much smaller crystallite sizes and correspondingly larger surface/volume ratios. Supports also function to stabilize small Group VIII-metal crystallites against thermal sintering. The heretofore disadvantage of well-dispersed supported Group VIII-metal catalysts is the extreme difficulty of effectively promoting such catalysts with potassium (or rubidium). The difficulty arises since the added potassium (or rubidium) promoter has a tendency to preferentially affix itself to the high surface area support rather than to the small Group VIII-metal crystallites. This of course reduces greatly the promotional effect of potassium (or rubidium).
Typically, prior art catalysts are prepared by first impregnating a support with a Group VIII-metal salt. Following reduction of the Group VIII-metal salt a potassium (or rubidium salt) is then deposited. Conversely, a support may be first impregnated with a potassium salt (or rubidium salt) and then subsequently impregnated with a Group VIII-metal salt, and then reduced. Alternately, a potassium salt (or rubidium salt) and a Group VIII-metal salt may be co-impregnated onto a support and then reduced so as to produce a supported bimetallic K (or Rb)/Group VIII-metal catalyst system.